Bug 1484101 - Ensure DrawTargetTiled::PadEdges is called with region in device space. r=rhunt

In MultiTiledContentClient we can create a DrawTargetTiled with a
different origin than the layer we are painting. We must therefore
ensure when edge-padding that we provide the valid region in the draw
target's device-space rather than layer-space. Not doing so was
causing us to pad out in incorrect directions, causing visible seams.

Differential Revision: https://phabricator.services.mozilla.com/D3993

--HG--
extra : moz-landing-system : lando

gfx/2d/2D.h

@@ -1300,6 +1300,7 @@ public:
 
   /**
    * Performs an in-place edge padding operation.
+   * aRegion is specified in device space.
    */
   virtual void PadEdges(const IntRegion& aRegion);
 

gfx/layers/client/MultiTiledContentClient.cpp

@@ -270,7 +270,7 @@ void ClientMultiTiledLayerBuffer::Update(const nsIntRegion& newValidRegion,
 
       // Edge padding allows us to avoid resampling artifacts
       if (gfxPrefs::TileEdgePaddingEnabled() && mResolution == 1) {
-        drawTarget->PadEdges(newValidRegion);
+        drawTarget->PadEdges(newValidRegion.MovedBy(-mTilingOrigin));
       }
 
       // Reset